1. Field of the Invention
The present invention relates to an optical element, an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
In an optical scanning device for a multi-color image forming apparatus, plural photoconductors are irradiated with plural light beams, respectively, which are emitted from plural laser light sources and pass through a light deflection element for deflecting the traveling directions of the light beams and plural imaging optical elements for imaging the light beams, and each photoconductor is scanned with each light beam in response to the information of each color image so as to form an image corresponding to each color on each photoconductor.
Recently, an image output apparatus such as a digital copying machine and a laser printer which apparatus includes tandem optical systems has been put to practical use in order to comply with the requirements of speeding up and high quality image of a multi-color image forming apparatus. In a tandem optical system in which four photoconductor drums are arranged along the conveyance direction of an output paper sheet, the four photoconductor drums are simultaneously exposed to and scanned with four light beams and four color images developed by using developing machines for respective colors (yellow, magenta, cyan, and black) are superposed sequentially so as to form a color image.
In regard to an imaging optical element of an optical scanning device, JP-A-2003-177214 (particularly, see claim 1 and paragraph 0007) discloses a plastic optical element which is integrally constructed by laminating and jointing plural plastic optical elements, characterized in that a concave portion which is un-transcribed by a mold wall surface for forming a surface containing one or both joint surfaces of the plastic optical element, and at least one step surface (convex surface) are provided on the surface. In the technique disclosed in JP-A-2003-177214, when the plastic optical element is constructed by jointing and integrating optical elements made of a plastic, the precision of arrangement of the jointed optical elements is high and the joint surfaces of the optical elements are improved to be laminated and integrated at low cost.
Also, JP-A-2004-318024 (Particularly, see claim 1 and paragraph 0010) discloses an optical element with a surface having a certain radius of curvature and subjected to transcription, for light-scanning with a beam emitted from a light source and deflected in the main scanning directions, characterized by including a positioning surface for positioning the main scanning directions of the optical element which positioning surface is formed on both edge portions of the optical element in the main scanning directions, and a pair of upper and lower reference surfaces which are orthogonal to the positioning surface. The technique disclosed in JP-A-2004-318024 is to provide an optical element which is capable of attaining the high precision of the positioning thereof with respect to the scanning directions and of easily positioning of plural common optical elements used for the stepwise arrangement thereof, without extremely raising the precision of the optical element.
Herein, in the plastic optical element disclosed in JP-A-2003-177214 and the optical element disclosed in JP-A-2004-318024, steps or reference convex portions are provided on the central portion on a surface different from the optical surface of the (plastic) optical element and both edge portions of the surface different from the optical surface of the (plastic) optical element. It is considered that such positions of the steps and reference convex portions on the surface different from the optical surface of the (plastic) optical element could sufficiently ensure the precision of the arrangement of the (plastic) optical element having no deformation from the ideal form thereof. However, in the case of the (plastic) optical element is a (plastic) optical element having a deformation from the ideal form thereof which may be caused in a process for molding the (plastic) optical element, the precision of the arrangement of the (plastic) optical element may be able to be ensured sufficiently.
For example, the position of the step or reference convex portion provided on the (plastic) optical element may be shifted in the sub-scanning directions of the (plastic) optical element due to the profile deformation of the (plastic) optical element in the sub-scanning directions, and as a result, the optical axis of the (plastic) optical element may be decentered. Accordingly, in regard to the beam spot of a light bean imaged via the (plastic) optical element, a desired bean spot diameter may not be able to be obtained and/or a desired beam spot position may not be able to be obtained (a curve of the scanning line may be caused). Thus, the optical quality of the (plastic) optical element may be degraded.
Also, in an optical scanning device for a multi-color image, the positions of the beam spots of light beams for respective colors may be displaces from one another (the curves of scanning lines of the light beams for respective colors which are different from one another may be caused) due to the profile deformation of the (plastic) optical element, and a significant color shift may be caused on an image formed in an image forming apparatus.
Such profile deformation of the (plastic) optical element tends to be caused when the cycle time period for molding the (plastic) optical element is reduced for the cost down of molding of the (plastic) optical element. That is, it is considered that the profile deformation of the (plastic) optical element may be caused by the non-uniformity of the temperature distribution of a mold for molding the (plastic) optical element or the non-uniformity of a cooling process after releasing the (plastic) optical element from a mold, which non-uniformity may be caused when the cycle time period for molding the (plastic) optical element is reduced. Therefore, differences among the profiles of the (plastic) optical elements may be produced, for example, among the shots of molding of the (plastic) optical elements and/or among the plural cavities for shaping the (plastic) optical elements formed in a mold.
Herein, the inventors have found to provide an optical element capable of improving the precision of arrangement of the optical element. Also, the inventors have found to provide an optical scanning device which includes an optical element capable of improving the precision of arrangement of the optical element. Furthermore, the inventors have found to provide an image forming apparatus which includes an optical scanning device which includes an optical element capable of improving the precision of arrangement of the optical element.